Process for preparing low formaldehyde polyacetal containing coating

ABSTRACT

The instant invention involves a process for forming a low formaldehyde, essentially color-free coating composition by applying the composition preferably to a wood substrate and curing. The composition comprises a blend of a polyhydroxy-containing polymer, an hexaalkoxymethylolmelamine, a polyacetal having the formula: ##STR1## wherein R is C 1  -C 3  and A is C 1  -C 20  allyl or aryl and an acid catalyst. The coating compositions of the instant invention exhibit extremely low color and give off a significantly reduced amount of formaldehyde when cured in comparison to conventional prior art, acid catalyzed melamine/formaldehyde cured coatings.

This is a divisional of copending application Ser. No. 07/612,261 filedon Nov. 9, 1990 and now U.S. Pat. No. 5,102,954.

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates to coating compositions. More particularly, thisinvention relates to melamine formaldehyde cured, coating compositionswhich give off reduced levels of formaldehyde.

2. Prior Art

In recent Years increasing concerns have been expressed about coatingcompositions which give off formaldehyde during the curing process.Therefore, there has been an emphasis on developing low formaldehyde orformaldehyde-free coatings.

U.S. Pat. Nos. 4,663,410 and 4,674,611, as well as European Patent201,693 and Chapter 31 of the American Chemical Society, ACS SymposiumSeries 367, Cross-Linked Polymers, Chemistry, Properties & Applications,disclose the reaction of a polyacetal with an acrylic resin to producean acetalated acrylic resin which then may be cross-linked by using apolyhydroxy containing monomer.

A second approach disclosed in U.S. Pat. No. 4,655,841 involves thesurface curing of a sizing composition by spraying a diacetal onto apolyhydroxylated resin which is based upon polyvinyl alcohol andpolyvinyl acetate.

Yet another approach disclosed in the article includes mixing anhydroxyl polymer with a polyacetal and then self-polymerizing theresulting product in situ.

An inherent disadvantage of many of the prior art polyacetal polymers isthat they are color formers unless used in the manner described herein.Accordingly, when acetal/OH reactions are carried out using productswhich do not conform to the chemical structures described herein,undesirable color formation is possible.

Therefore, it is an object of this invention to prepare compositionswhich give off reduced amounts of formaldehyde when they cure.

It is another object of this invention to prepare coating compositionsbased upon hydroxy polymers and monomeric polyacetals.

It is a further object of this invention to prepare coatings based uponpolyacetal cross-linking agents which do not form undesirably discoloredcoatings.

These and other objectives are obtained by carrying out the process ofthe instant invention using the products described herein.

SUMMARY OF INVENTION

Basically, the instant invention involves forming an essentiallycolor-free, low formaldehyde, coating composition by applying a mixtureof a polyhydroxy-containing polymer, a hexaalkoxymethylolmelamine, apolyacetal and an acid catalyst to a substrate and curing the coatingcomposition.

DETAILED DESCRIPTION OF INVENTION

The polyhydroxy-containing polymers which may be used in the instantinvention may be selected from a wide variety of materials, includingconventional, well-known alkyd resins, polyester resins, acrylics, vinylpolymers and copolymers, cellulose acetate based materials and aliphatichydroxy containing epoxy resins. The key requirement of any of thesematerials is that they have sufficient reactive aliphatic hydroxylgroups present to enter into a crosslinking reaction with amelamine/formaldehyde resin of the conventional type which is present inconventional amounts.

Alkyd resins, for the most part, are polymers prepared by reactionsbetween fatty acids or alcoholized oils and mono and poly carboxylicacids and possibly other polyols. Polyester resins are similar exceptthat no fatty acids or oil-based materials are made part of the polymer.Regardless of which ingredients are chosen, however, sufficient freealiphatic hydroxyl groups must be present to ensure that the alkyd resinwill crosslink and provide a cured solvent-resistant coating.

The acrylic and vinyl polymers and copolymers are prepared generally bycopolymerizing acrylic or vinyl monomers in conventional amountsutilizing preferably free radical generating catalysts. As with thealkyd/polyester polymers of the instant invention, it is essential thatsufficient hydroxyl groups be present on the polymeric backbone toensure adequate crosslinking. Hydroxyl groups are generally admitted topolymeric backbone by the use of hydroxy-containing acrylic or vinylmonomers as described hereafter. Among the various monomers which may beutilized to form the acrylic or vinyl polymers are included acrylic andmethacrylic acids and their esters, including their hydroxy esters suchas hydroxypropyl acrylate or methacrylate and hydroxyethylacrylate ormethacrylate, aryl based monomers such as styrene, vinyl toluene, othermonomers such as allyl alcohol and a wide variety of other conventionalmonomers. Monomers which interferingly react with the melamineformaldehyde/hydroxyl crosslinking mechanism or thepolyacetal/transetherification reaction should not be utilized.

The cellulosic acid esters are of the type which is well known in thecoating, particularly wood coating industry. Particularly preferred arethe cellulose acetates and the mixed cellulose acetate carboxylates suchas cellulose acetate butyrate. Again, there must be sufficient free andunreacted hydroxyl groups present in the cellulosic polymer to permitcrosslinking of the polyhydroxy containing material in question.

The second key ingredient of the low formaldehyde low color coatingcompositions of the instant invention is an hexaalkoxymethylolmelaminewherein the alkoxy group contains up to about 3 carbon atoms, ormixtures thereof. Preferably the melamine is or is the equivalent of anhexamethoxymethololmelamine of the Cymel 300, 301 family manufactured byAmerican Cyanamid Company. Stating that this material is anhexaalkoxymethylolmelamine does not mean that there are precisely sixalkoxy groups on each molecule. Materials in which there are on theaverage more than five alkoxy groups, i.e., 5.1 up to 6, would qualifyas an "hexaalkoxymethylolmelamine" as that term is employed herein.

Another important ingredient of the coating composition of the instantinvention is a polyacetal having the general formula: ##STR2## whereineach R may be the same or different and contains from one to threecarbon atoms and A is C₁ -C₂₀ alkyl or aryl moiety. It is essential inselecting the polyacetal for use in the instant invention that the Amoiety not contain any groups which are interferingly reacting with theacetal transetherification reaction or the melamine/formaldehydecrosslinking reaction. Preferably, the polyacetal is an alkyl polyacetalwherein A contains from 1-10 carbon atoms and the R groups are methyl.

The final crucial ingredient of the composition of the instant inventionis an acid catalyst, preferably a sulfonic acid and most preferablyparatoluene sulfonic acid.

The various constituents of the composition of the instant inventionshould be added in the following solids weight percent ranges, the totalbeing 100.

    ______________________________________                                                          Permitted                                                                             Preferred                                           ______________________________________                                        Polyhydroxy polymer 55-80     63-75                                           Hexaalkoxymethylolmelamine                                                                        10-25     11-21                                           Polyacetal           2-12      3-10                                           Acid Catalyst       0.25-10   2-6                                             ______________________________________                                    

The compositions of the instant invention may be modified by theaddition of other ingredients typically utilized in coatingcompositions, particularly in wood coating compositions. Included amongsuch ingredients are conventional amounts of fillers, reinforcingagents, flow, slip and mar control agents and pigments. The onlyrequirement is that none of these ingredients may interferingly reactwith any of the essential constituents of the instant invention.

The compositions of the instant invention are blended together utilizingconventional blending and agitation methods employed in the coatingsindustry.

The coating compositions of the instant invention may be applied to anysubstrate such as wood, metal, plastic and glass, although woodsubstrates are preferred. The compositions may be used alone or incombination with other sanding coats, sealer coats, primer coats and topcoats.

The coating compositions of the instant invention are most useful inclear coating applications where low Yellowing is preferred although thesame technology can be applied to pigmented coatings of varyingopacities and colors.

As pointed out above, the coating compositions of the instant inventionfind particular utility as wood coating compositions in applicationswhere a reduction in the level of formaldehyde is desired. One of themajor goals in the coatings industry, particularly as formaldehyde hasbeen identified by some as an undesirable byproduct, has been toeliminate or reduce the amount of formaldehyde which is given off duringcuring. Formaldehyde is most often generated by crosslinking agents suchas urea/formaldehyde and melamine/formaldehyde resins. The coatingcompositions of the instant invention provide a means whereby theformaldehyde generated by the crosslinking or curing ofhexaalkoxymethololmelamine materials can be reduced by 25 percent ormore.

At the same time, the coating compositions of the instant invention areparticularly useful because they do not exhibit the adverse colorforming characteristics typical of compositions which utilizepolyacetals as a part of the polymeric backbone.

EXAMPLE 1

A curable coating composition was prepared by agitating the following:108.2 parts of RJ 100 a styrene allyl alcohol copolymer available fromthe Monsanto Company, 43.3 parts of Tetronic 901, a reactive polyol(propylene oxide derivative) available from the BASF WyandotteCorporation, 32.5 parts of Cymel 303, a hexamethoxymethylolmelamineavailable from the American Cyanamide Company, 0.5 parts of Byk 300 aflow and mar control agent available from Byk-Chemie USA, 0.6 parts ofByk 320, another flow control agent, 130.0 parts of toluene, 343.0 partsof xylene, 50.0 parts of butanol, 60.5 parts of butyl acetate, 213.4parts of Solvesso 100 solvent from the Exxon Co., 10.0 parts ofmethylamyl ketone and 8.0 parts of 1, 1, 3, 3-tetramethoxypropane. Themixture was catalyzed by adding 2 percent by volume of a 60% by weightparatoluene sulfonic acid solution in a blend of methyl and isopropylalcohols.

The above-described coating composition was applied by electrostaticdisk to kitchen cabinet doors which had been precoated with aconventional vinyl modified alkyd urea formaldehyde sealer which hadbeen applied to a wet film thickness of about 2.3 to about 2.7 mils andallowed to flash for 25 minutes at ambient temperature before beingexposed to a bake cycle of 4 minutes at 150° F. followed by a cool downcycle of 6 minutes at ambient temperature. The sealer coat was sandedand the composition described above was then applied to a wet filmthickness of about 3.1 to about 4.3 mils. This coating was allowed toflash for 17 minutes at ambient temperature before baking for 4 minutesat 150° F. followed by a cool down cycle of 14 minutes at ambienttemperatures. The resulting coating was evaluated in accordance withASTM 161:1-1980 along with x-hatch/tapepull adhesion test, a nickelscratch test and a fingernail (leading edge) mar resistance test.Satisfactory coating properties were obtained.

EXAMPLE 2

The following were mixed under agitation: 21.1 parts of xylene, 59.8parts of Cymel 303 as described in Example 1, 42.0 parts of anisobutylated urea, AM1023 available from Akzo Coatings, Inc., 387.2parts of a coconut alkyd known as Relrez AL4136E560, which is availablefrom Akzo Coatings, Inc., 3.0 parts of a silica flattening agent, 24.7parts of 1,1,3,3-tetramethoxypropane and 81.7 parts of isobutanol. Thefollowing were added to the above mixture under agitation: 180.1 partsof toluene, 46.0 parts of isopropyl alcohol, 2.0 parts of a mar controladditive and 0.5 parts of Byk 300 as described in Example 1. Aftermixing the above, 149.4 parts of a 20% solution intoluene/isobutanol/methylisobutylketone of Eastman Chemical Products,Inc.'s CAB 553-04 and 0.16 parts of a silica flattening agent wereadded. Prior to application, a PTSA solution was added at a level of 4%PTSA on coating vehicle solids.

The above-described coating composition was applied by air assistedairless spray to kitchen cabinet doors which had been precoated with asealer containing tetramethoxypropane. The tetramethoxypropane sealerwas comprised of 4.82 parts of the isobutylated urea AM1023 describedabove, 16.5 parts of the Cymel 303, described above, 50.61 parts of thecoconut alkyd Relrez AL4136E560 described above, 21.5 parts of the CAB553-04 described above, 6.09 parts of 1,1,3,3-tetramethoxypropane, 0.81part of a polypropylene wax, 0.01 part of a silica flattening agent, and0.01 part of a defoamer. The sealer was applied to a wet film thicknessof about 4.0 to about 5.0 mils and allowed to flash for 9 minutes atambient temperature before being exposed to a bake cycle of 3 minutes at98°-110° F. and then to a bake cycle of 2.25 minutes at 170° F. followedby a cool down cycle of 8.25 minutes at ambient temperature. The sealercoat was sanded and the composition described above was then applied toa wet film thickness of about 5.0 to about 7.0 mils. This coating wasallowed to flash for 12 minutes at ambient temperature before beingexposed to a bake cycle of 9.75 minutes at 97° F. and then a bake cycleof 2.25 minutes at 155° F. followed by a cool down cycle at ambienttemperature. Satisfactory coating properties were obtained.

EXAMPLE 3

Example 2 was repeated except that no cellulose acetate butyrate wasadded to the mixture. Satisfactory coating properties were obtained.

EVALUATION

The compositions of the instant invention, which were applied to kitchencabinet doors and then cured, exhibited a 35-52 percent reduction in theemission of formaldehyde during application and curing, compared toconventional prior art wood coating compositions as monitored by aBucharach TWA-lot number 5072T over a four-hour period at ambienttemperature and a relative humidity of about 63%.

We claim:
 1. A process for forming a coating composition which comprisesblending a polyhydroxy-containing polymer, an hexaalkoxymethylolmelaminewherein the alkoxy group is C₁ -C₃, or mixtures thereof, a polyacetalhaving the general formula: ##STR3## wherein R is C₁ -C₃ alkyl, ormixtures thereof, and A is selected from alkyl and aryl groups havingfrom 1-20 carbon atoms and no other groups interferingly reactive withthe acetal transetherification reaction or thehexaalkoxymethylolmelamine crosslinking reaction and an acid catalyst.2. The process of claim 1 wherein the polyhydroxy-containing polymer isselected from vinyl polymers, alkyds, polyester's and cellulosic esters,wherein said polyhydroxy-containing polymer contains unreacted aliphatichydroxyl groups.
 3. The process of claim 1 wherein R is CH₃ and A is C₁-C₁₀ alkyl.
 4. The process of claim 2 wherein the polyhydroxy containingpolymer is a styrene/allyl alcohol copolymer.
 5. The process of claim 1wherein the hexaalkoxymethylolmelamine is a hexamethoxymethylolmelamine.6. The process of claim 1 wherein the acid catalyst is a sulfonic acid.7. The process of claim 6 wherein the sulfonic acid is paratoluenesulfonic acid.
 8. The process of claim 1 wherein thepolyhydroxy-containing polymer is present at about the 55 to 80 percentby weight level, the hexaalkoxymethylolmelamine is present at about the10 to 25 percent by weight level, the polyacetal is present at about the2 to 12 percent by weight level and the acid catalyst is present atabout the 0.25 to 10 percent by weight level.
 9. The process of claim 1wherein the polyhydroxy-containing polymer is present at about the 63 to75 percent by weight level, the hexaalkoxymethylolmelamine is present atabout the 11 to 21 percent by weight level, the polyacetal is present atabout the 3 to 10 percent by weight level and the acid catalyst ispresent at about the 2 to 6 percent by weight level.
 10. A process forforming a coating composition which comprises blending apolyhydroxy-containing polymer selected from the group consisting ofhydroxy-containing cellulosic acetate butyrates, alkyds, polyesters andvinyl polymers; an hexaalkoxymethylolmelamine wherein the alkoxy groupis C₁ -C₃ or mixtures thereof; a polyacetal having the general formula:##STR4## wherein A is C₁ -C₁₀ alkyl and contains no other groupsinterferingly reactive with the acetal transetherification reaction orthe hexaalkoxymethylolmelamine crosslinking reaction; and a sulfonicacid.